1. Field of the Invention
The present invention relates to a probe card for connecting a wafer to be measured with a measuring device in a semiconductor wafer testing device.
2. Description of the Related Art
Marked progress has been seen in the recent semiconductor manufacturing technology, including the progress of fine processing technology required for high density integration. For high density implementation as well as for suppression of heat generation and downsizing each elements, the current and voltage required for operation in the semiconductor devices are reduced year by year. Thus, the importance of microcurrent measurement has been recognized, and higher accuracies in microcurrent measurement are required in probe cards for connecting measuring devices and wafers.
Such a requirement is also present in the wafer process control testing. As a conventional testing method for process control, measurement chips referred to as TEG (Test Element Group) are formed at the same time as ordinary chips are formed on wafers for characteristic measurements. In such measurement, a probe card is used as an interface card between a measuring device and a wafer prober for moving and positioning a wafer to be measured, and is provided around its circular hole with a plurality of wafer contact needles also referred to as contact blades so as establish contacts with measuring terminals of the wafer.
In FIG. 10, a probe card is generally a circular disk 100 having a diameter from about 20 to 30 cm, with its central portion provided with an opening 102 through which contact needles or blades (not shown) can extend to contact with measuring terminals on a semiconductor wafer placed to be measured just under the probe card. Each wire 103 extends from each needle or blade located at the inner periphery toward the outer periphery where it is connected to a measuring device or the like. That is, a signal picked up from each terminal of a semiconductor wafer to be measured is introduced into the outer periphery of the probe card via each needle or blade, and is then sent to a measurement device from an external connection terminal 104 located on the outer periphery of the probe card. At that time, wires for connecting the semiconductor wafer to be measured with the measuring device are several tens in number and placed adjacent to one another. Guard patterns 105 formed with a conductor are located around the corresponding external connection terminals 104. In the example shown in the drawing, the number of external connection terminals or pins is forty eight (48). Such a probe card is also known as a personality board or interface card. See Japanese Utility Model Provisional Publication (JP-A) No. Showa 64-47042, and Japanese Patent Provisional Publication (JP-A) No. Heisei 8-330369.
In general, such a probe card has external contact terminals 104 at its outer periphery for connection with a measuring device. Each contact terminal 104 is composed of a planar conductor provided on a substrate surface of the probe card, on which, for example, a conductor contact needle, such as a contact pin member which comprises a conductive rod with a circular cross-section provided with a spring at its base and surrounded by a cylindrical member such that on application of appropriate force the rod is retracted and biased by a reaction force so as to establish electrical contact by pushing the tip of the rod against a conductor surface, with a mechanism to provide secured electric contact is pushed against the planar conductor surface of the probe card, thereby to connect with an external apparatus such as a measuring device. Further, each external contact terminal 104 has a pattern extension portion 106 near its inner periphery, and a corresponding planar conductor on the surface of the probe card extends inwardly. To the pattern extension portion one end of a coaxial cable 103 is connected, and the other end of the coaxial cable is connected with a base end of a contact needle which has the shape of the needle or blade as described above. Namely, the coaxial cable 103 extends in the air between the pattern extension portion and the contact needle. The needle or blade is attached to the inner periphery of the probe card and extends inwardly therefrom. Further, the tip of the needle or blade extending inwardly is connected to a predetermined terminal on a semiconductor wafer to be tested.
As mentioned above, in the field of testing semiconductor elements, testing appliances which can measure microcurrents smaller than the currently used level are required. Also, the measuring accuracy of femtoampere order is required for interface cards used for wafer probers. Major problems on developing testing devices with such high performance include leakage current between adjoining electric wires for connecting a testing device and a semiconductor to be measured and dielectric absorption occurring between such wires and dielectrics in a probe card substrate.
For example, the problem of dielectric absorption occurs due to dielectric absorption properties (absorption current) of the dielectric used as the probe card substrate. Generally, insulating materials show dielectric polarization when a voltage applied across two electrodes changes, and absorb a current gradually until the polarization process is completed. Therefore, even if a predetermined voltage is applied to a semiconductor wafer for current measurement, the current measurement can not be performed correctly for a certain time period and a waiting time is necessary until the current flow becomes stabilized. A waiting time is also necessary when the voltage supply is terminated because a discharge current flows out gradually. For conventional interface cards for wafer probers, it is not unusual that such a waiting time for measurement may be several tens of seconds until the dielectric absorption current is reduced to a femtoampere order. This is one of important problems in case of reducing the time required for the microcurrent measurement.
As the measuring accuracy of a femtoampere order is now required, the leakage current flowing through a dielectric material between adjoining wires has also become important.
Among the components on the probe card as mentioned above, portions of the external contact terminal and the pattern extension portion tend to be affected by the dielectric absorption and current leakage. The coaxial cable extending from each pattern extension portion runs in the air so that it is little influenced by the dielectric absorption or current leakage. As the needle portion, for example, a coaxial highly insulating needle can be used, and a shield wire surrounding the core wire of this needle can be connected with a guard. In addition, insulation between adjoining needles having a portion of the needle wire not covered with the shield wire is good because it is achieved by air which has the same dielectric constant approximately as that of vacuum. Also, the response time of the dielectric polarization is not an issue.
To address these problems caused by the external contact terminal, conventionally, a plurality of through-holes are formed from electrodes on the surface through the probe card substrate such that the periphery of the electrodes can be defined. One example of such technologies is described in Japanese Patent Provisional Publication No. Heisei 8-335754.
One problem in such a method is that a distortion tends to occur in the probe card as the number of through-holes is increases, and that the physical strength of the card degrades in or near the region of the through-holes. Even if the number of the through-holes is increased regardless of manufacturing difficulties and cost elevation, some leakage current through the dielectric between these through-holes still remains as a problem, and a dielectric loss can not be reduced to a desired level as a higher level of measuring accuracy is required.
It has been noted that there are variations in the dielectric and leakage current properties of each wire for each probe card. Therefore, even if the manufacturing process control is improved sufficiently, such variations in the dielectric and current leakage properties of each wire for each probe card would limit the accuracy of current measurements. For example, in case of a currently used probe card with 48 pins after application of a voltage of about 10 volts, the leakage current 10 seconds after that voltage application of most pins is about 0.3×10−13 A. However, such a leakage current of other specific pins becomes about 1 to 2×10−13 A.